Passive optical network (PON) is a widely used technology for residential and business broadband access. PONs are considered to be inexpensive for network operators because they do not require any active equipment or power supplies between the operator's central office (CO) and customer's premises (CP). As shown in FIG. 1, downstream PON traffic is directed from the Optical Line Terminal (OLT) 101 residing in the CO towards a number of Optical Network Terminals (ONT) 110, 111, 112, 113 residing on the CP end 103. A power splitter 106 directs traffic to the individual ONTs.
Since the OLT 101 is the only unit transmitting in the downstream direction, there can be no collision between downstream-bound packets. Upstream PON traffic shares the same optical fiber with the downstream traffic, utilizing a different wavelength. Therefore, there cannot be any collision between downstream and upstream packets either. However, since the upstream traffic originates from all ONTs and all ONTs are transmitting on the same wavelength, packet collision can occur if two or more ONTs are transmitting simultaneously. In order to prevent collisions, upstream PON traffic is managed in the Time Division Multiple Access (TDMA) fashion. One of the functions of the OLT 101 is to schedule and grant separate time slots to each ONT, thus avoiding collision between upstream packets. Transmitter lasers of each ONT can be turned on only during their respective transmission time slots.
The OLT 101 must be capable of receiving bursts of data from different ONTs. The structure of a typical OLT 101 is shown in greater detail in FIG. 2 and includes an OLT digital chip 201 and an OLT optics module 205. Downstream signals from OLT MAC (tx) 202 are serialized on the OLT digital chip 201 by serializer 203. The downstream signals 213 then pass to laser driver and laser 206 of the OLT optics module 205, then through WDM filter 207 to be transmitted downstream 219 to power splitter 106. Upstream signals are received by a burst-mode receiver which includes a photo detector (PD) 210, transimpedance amplifier (TIA) 209 and limiting amplifier (LA) 208 of the OLT optics module 205 and data recovery (CDR) circuitry 204 of the OLT digital chip 201. The PD 210 performs conversion of the received optical signal into an electrical signal. The TIA 209 and LA 208 restore the electrical signal to a standard digital voltage level, and the CDR 204 extracts the transmitted data contents from the LA output signal 214.
The standard upstream bit rates have been steadily going up from the initial 155 Mb/s in APON in the mid-1990s, to 1.25 Gb/s in Gigabit-capable PON (GPON) [ITU-T G.984] and Ethernet PON (EPON) [IEEE 802.3ah] of mid-2000s and are likely to reach 10 Gb/s in the early 2010s. The high bit rates pose an increasing challenge for implementation of the burst-mode receiver, particularly of its analog circuits. Particularly, it is very difficult to design the TIA 209 and LA 208 that can restore the received signal fast enough and without distortion of its duty cycle, while supporting a wide dynamic range of the input signal.
Among various burst-mode CDR methods, oversampling CDR architectures appear to be particularly practical because they do not require the receiver PLL to lock on the frequency and phase of the transmitter's clock. Additionally, oversampling CDRs are convenient for implementation because they are almost completely based on digital circuits.
However, in existing systems, the TIA, LA and CDR are optimized separately and don't “talk” to each other. As the bit rates go up, the analog circuits available for TIA and LA implementation become less efficient, hard to design and tune and typically not fast enough to meet the standard specification. This can result in the loss of entire upstream bursts due to the OLT's failure to detect the delimiter, or in the loss of individual packets due to the increased bit error rate caused by the distorted signal at the LA output. Mitigation of this problem by increasing the preambles and margins between bursts, results in lower bandwidth utilization.
What is required is a system that uses available information to adjust network performance and/or alleviate network conditions.